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Flue Gas Desulphurisation (FGD)

Summary

Flue gas desulphurisation (FGD) is a technology used to remove sulphur dioxide (SO2) from the exhaust flue gases of fossil fuel power plants. Fossil fuel power plants burn coal or oil to produce steam for steam turbines, which in turn drive electricity generators. SO2 is a transboundary air pollutant, partly responsible for forming acid rain. Tall flue gas stacks disperse emissions by diluting the pollutants in ambient air and transporting them to other regions.

There are a variety of methods now available to facilitate FGD. The most common include:

Wet scrubbing

Spray-dry scrubbing

Wet sulphuric acid process

SNOX flue gas desulphurisation

Dry sorbent injection systems

Implementation

In Europe, FGD is implemented in some cases through the Large Combustion Plant Directive (2001/80/EC), the purpose of which is to limit the amount of sulphur dioxide, nitrogen oxides and dust emitted from large combustion plants each year.

Impact

For a typical coal-fired power station, FGD will remove 95% or more of the SO2 in the flue gases. However, this varies depending on the specific technology. For example, it is estimated that wet scrubbers can remove more than 90% of SO2 emissions, while less than 80% is removed from dry scrubbers (i.e. the lowest efficiency rate). Though newer designs for dry scrubbers are capable of achieving efficiencies of approximately 90%.

The image below displays the features and chain of events of a wet FGD scrubber.

Costs & Benefits

The capital, operating and maintenance costs per short ton of SO2 removed (in 2001 US dollars) are:

For wet scrubbers larger than 400 MW, the cost is $200 to $500 per ton

For wet scrubbers smaller than 400 MW, the cost is $500 to $5,000 per ton

For spray dry scrubbers larger than 200 MW, the cost is $150 to $300 per ton

For spray dry scrubbers smaller than 200 MW, the cost is $500 to $4,000 per ton

Though expensive, these costs have decreased by more than 30% since the 1990s. Furthermore, wet systems generate a wet waste product and may result in a visible plume, and the disposal of waste products significantly increases operating and maintenance costs (which are already relatively high).

Evidence & Reference

Modelling this Measure

In terms of modelling this measure, the GAINS model has the ability to address the uptake of this technology within power plants. The broad approach is to define specific power plants in the region, their associated activity levels, and to estimate emissions based on the specific FGD technology in place including the respective removal efficiency. The FGD performance as an abatement technology is estimated in applied research and reformatted for use in the specific modelling framework.